Question

I’m working on a mathematical model that uses data generated from XFOIL, a popular aerospace tool used to find the lift and drag coefficients on airfoils.

I have a Perl script that calls XFOIL repeatedly with different input parameters to generate the data I need. I need XFOIL to run 5,600 times, at around 100 seconds per run, soabout 6.5 days to complete.

I have a quad-core machine, but my experience as a programmer is limited, and I really only know how to use basic Perl.

I would like to run four instances of XFOIL at a time, all on their own core. Something like this:

while ( 1 ) {

    for ( i = 1..4 ) {

        if ( ! exists XFOIL_instance(i) ) {

            start_new_XFOIL_instance(i, input_parameter_list);
        }
    }
} 

So the program is checking (or preferably sleeping) until an XFOIL instance is free, when we can start a new instance with the new input parameter list.

Answer 1

Try Parallel::ForkManager. It’s a module that provides a simple interface for forking off processes like this.

Here’s some example code:

#!/usr/bin/perl

use strict;
use warnings;
use Parallel::ForkManager;

my @input_parameter_list = 
    map { join '_', ('param', $_) }
    ( 1 .. 15 );

my $n_processes = 4;
my $pm = Parallel::ForkManager->new( $n_processes );
for my $i ( 1 .. $n_processes ) {
    $pm->start and next;

    my $count = 0;
    foreach my $param_set (@input_parameter_list) {         
        $count++;
        if ( ( $count % $i ) == 0 ) {
            if ( !output_exists($param_set) ) {
                start_new_XFOIL_instance($param_set);
            }
        }
    }

    $pm->finish;
}
$pm->wait_all_children;

sub output_exists {
    my $param_set = shift;
    return ( -f "$param_set.out" );
}

sub start_new_XFOIL_instance {
    my $param_set = shift;
    print "starting XFOIL instance with parameters $param_set!\n";
    sleep( 5 );
    touch( "$param_set.out" );
    print "finished run with parameters $param_set!\n";
}

sub touch {
    my $fn = shift;
    open FILE, ">$fn" or die $!;
    close FILE or die $!;
}

You’ll need to supply your own implementations for the start_new_XFOIL_instance and the output_exists functions, and you’ll also want to define your own sets of parameters to pass to XFOIL.